Swarming is a widespread mode of surface colonization by flagellated bacteria. It shares features with other surface phenomenon such as biofilm formation and host invasion, and is therefore a particularly relevant model for uncovering and understanding bacterial surface-sensing mechanisms. Swarmer cells generally make more flagella, and excrete surfactants and polysaccharides that aid surface motility by generating wetness and providing lubrication. Our studies in this grant period have established that external wetness is more critical for swarming in S. typhimurium than increased production of flagella. Studies with swarming-defective mutants in the chemotaxis signaling pathway have led to the surprising discovery that the flagellum itself is involved in both generating and sensing wetness to control its own biogenesis and movement. Our data support a model in which information about how wet the environment is (and hence how conducive to swarming) is conveyed to the flagellar Type III secretion system to control export of a negative regular that controls the last and energetically most costly step in flagellar biosynthesis. Switching of the flagellar motor is implicated in generating wetness. Interestingly, the 'wetness' signal is conveyed via the flagellar system to the SPI-1 virulence system, which specifies the Needle structure responsible for injecting virulence factors into host cells. We have also discovered that fliL, whose function has been a mystery, is essential only for surface motility; fliL is the first gene in a flagellar operon dedicated to synthesis of the switch and the Type III export complex. We have uncovered three new players in the chemotaxis system that appear to work together to promote surface colonization. The proposed work aims to understand (1) the mechanism by which the flagellum generates and senses wetness, (2) the role of FliL in surface motility, (3) the regulatory circuits that connect the motility and virulence systems, as well as (4) the novel functional association between the three new chemotaxis genes. Our studies are expected to lead to a unified understanding of the regulation of surface motility in a variety of bacterial species, as well as common principles that govern the secretion of flagellar components and virulence factors, both of which are regulated by surface contact. PUBLIC HEALTH RELEVANCE: Swarming is a widespread mode of surface colonization by flagellated bacteria. It shares features with other surface phenomenon such as biofilm formation and host invasion, and is therefore a particularly relevant model for uncovering and understanding bacterial surface-sensing mechanisms. Our studies are expected to lead to a unified understanding of the regulation of surface motility in a variety of bacterial species, to an elucidation of common principles that govern the secretion of flagellar components and virulence factors exported through the Needle complex, and to uncovering new mechanisms by which external water content is gauged by these nanomachines.